Process for preparing a polypropylene composition

ABSTRACT

The invention relates to a process for producing a polypropylene composition having an improved balanced combination of high flowability, high stiffness and impact, and high level of optical properties. Further, the invention relates to the polypropylene composition having the above-mentioned properties, as well as an article comprising the polypropylene composition.

The present invention relates to a process for producing a polypropylenecomposition. More specifically, the invention relates to a process forproducing a polypropylene composition comprising propylene and one ormore comonomers selected from ethylene and C₄-C₁₀ alpha-olefins and tothe polypropylene composition obtained by said process. The inventionfurther relates to an article comprising the polypropylene composition.

Propylene homopolymers and copolymers are suitable for many applicationssuch as packaging, textile, automotive and pipe. An important area ofapplication of propylene homopolymers and copolymers is the packagingindustry, particularly in film and moulding applications.

In the field of packaging it is of great importance to have a goodflowing polypropylene composition with good mechanical properties, i.e.a high tensile modulus and good impact strength. The good flowability isneeded for achieving a good processability in various manufacturingprocesses of articles, like for example in injection moulding processes,thereby allowing a high production speed, which is generally required inmass production market. Mechanical properties are also important in thistype of applications, particularly in the field of containers, where itis needed to hold the content, such as food or fluid, contained therein.Additionally, there is the need to have sufficient stiffness for thecontainer to be stacked.

Additionally, the polypropylene composition should also withstandmechanical compression damage, which is frequently incurred by e.g.dropping the articles.

Still further, also the haze should be acceptable. Particularly, a goodbalance between stiffness and haze is needed.

However, at least some of these properties may only be achieved at theexpense of other of these properties. For instance, with increase ofmelt flow rate the stiffness can be improved, while the impactproperties significantly drop. Thus, impact strength and melt flow rateof the polypropylene composition behave in a conflicting manner.

Further, a high degree of crystallinity of the polypropylene compositionrenders it rather stiff, however it also increases its haze. Thus, thebalance of stiffness and haze in the polypropylene composition is ofgreat importance.

Thus, there is a general need of a process for the manufacture of apolypropylene composition which is featured by a balanced combination ofhigh flowability, high stiffness and impact, and high level of opticalproperties (low haze value).

EP2539398 discloses a process for the preparation of a random propylenecopolymer in a sequential polymerization process, wherein said processcomprises the steps of

-   -   a) polymerizing propylene and at least one ethylene and/or a C₄        to C₂₀ α-olefin in a first reactor (R1) obtaining the a        polypropylene (PP1) being the a random propylene copolymer        (R-PP1), said polypropylene (PP 1) has a melt flow rate MFR₁₀        (230 C) measured according to ISO 1133 of not more than 1.5 g/10        min,    -   b) transferring the first polypropylene (PP1) in a second        reactor (R2)    -   c) polymerizing in the second reactor (R2) and in the presence        of said first polypropylene (PP1) propylene and optionally at        least one ethylene and/or a C₄ to C₂₀ α-olefin obtaining thereby        a second polypropylene (PP2), said first polypropylene (PP1) and        said second polypropylene (PP2) form a (intimate) mixture and        said second polypropylene (PP2) being a first propylene        homopolymer (H-PP1) or a second random propylene copolymer        (R-PP2),    -   d) transferring the mixture of the first polypropylene (PP1) and        the second polypropylene (PP2) in a third reactor (R3), and    -   e) polymerizing in the third reactor (R3) and in the presence of        the mixture of the first polypropylene (PP1) and the second        polypropylene (PP2) propylene and optionally at least one        ethylene and/or a C₄ to C₂₀ α-olefin obtaining thereby a third        polypropylene (PP3), said third polypropylene (PP3) being a        second propylene homopolymer (H-PP2) or a third random propylene        copolymer (R-PP3) and the first polypropylene (PP1), the second        polypropylene (PP2) and the third polypropylene (PP3) form a        (intimate) mixture.

However, the invention in EP2539398 is directed to pipes, thus, MFR₂ ofthe resulting random propylene copolymer is between 0.5 to 10.0 g/10 minand may comprise in particular beta-nucleating agents. Additionally,EP2539398 is silent about haze properties of the resulting randompropylene copolymer.

EP2338657 related to a heterophasic polypropylene composition withrather high melt flow rate, high stiffness, acceptable impact propertiesand an advantageous balance between stiffness and transparency. Thecomposition disclosed comprises 30-60 wt.-% of a propylene homopolymerfraction (A), 10-50 wt.-% of a propylene random copolymer fraction (B),1-20 wt.-% of a first elastomeric ethylene propylene copolymer fraction(C), 1-20 wt.-% of a second elastomeric ethylene propylene copolymerfraction (D), and 5-25 wt.-% of an ethylene homo- or copolymer fraction(E). The document does not disclose haze values on 2 mm plaques.Furthermore, the examples do not contain an alpha nucleating agent.Moreover, no pelletizing step is disclosed in Dl.WO2012/126759 relates to thermoplastic polyolefin compositions having agood balance of mechanical and optical properties. The propylene randomcopolymer composition comprises 60-85 wt.-% of a copolymer of propyleneand from 0.1 to 2 wt.-% of units derived from ethylene and 15-40 wt.-%of a copolymer of propylene and from 7 to 17 wt.-% of units derived fromethylene, said composition having a total ethylene content of from 3 to4.5 wt.-%, a melt flow rate value according to ISO 1133 (230/2.16) offrom 10 to 120 g/10 min 10 min. It does not disclose the presence of apropylene polymer (PP-c) with a comonomer content of 0.5-2.5 wt.-%.WO2016/116606 relates to bimodal polypropylene random copolymercompositions having a good balance of mechanical and optical properties.It differs from the present invention in that it does not comprise apropylene polymer (PP-c) with a comonomer content of 0.5-2.5 wt.-%.

The present invention is based on the finding that the above discussedneeds for a balanced combination of high flowability, high stiffness andimpact, and high level of optical properties (low haze value) can beachieved by a process for producing a specific polypropylenecomposition. Thus, the present invention provides a specific process forproducing a specific polypropylene composition, the process comprisingthe steps of:

-   -   a) polymerizing in a first reactor, preferably a slurry reactor,        monomers comprising propylene and optionally one or more        comonomers selected from ethylene and C₄-C₁₀ alpha-olefins, to        obtain a first propylene polymer fraction having a comonomer        content in the range of 0.0 to 1.0 wt %,    -   b) polymerizing in a second reactor, preferably a gas-phase        reactor, monomers comprising propylene and one or more        comonomers selected from ethylene and C₄-C₁₀ alpha-olefins, in        the presence of the first propylene polymer fraction to obtain a        second propylene polymer fraction having a comonomer content in        the range of from 4.5 to 20.0 wt %,    -   c) pelletizing the second propylene polymer fraction,    -   d) melt-mixing the pelletized second propylene polymer fraction        in the presence of        -   i. a propylene polymer (PP-c) containing one or more            comonomers selected from ethylene and C₄-C₁₀ alpha-olefins            wherein the comonomer content is in the range of from 0.5 to            2.5 wt % and        -   ii. at least one alpha-nucleating agent,    -   wherein the polypropylene composition has:        -   i—an MFR₂ in the range of from 11.0 to 60.0 g/10 min, as            measured according to ISO 1133 at 230° C. under a load of            2.16 kg,        -   ii—a haze value <20%, as measured according to ASTM D1003 on            injection moulded plaques having 1 mm thickness produced as            described in EN ISO 1873-2.

The first and the second propylene polymer fractions, according to thepresent invention, are produced in a sequential polymerization process,in the presence of an olefin polymerization catalyst.

The term “sequential polymerization process”, in the process forproducing the first and the second propylene polymer fractions,indicates that the propylene polymer fractions are produced in a processcomprising at least two reactors connected in series. In one preferredembodiment the term “sequential polymerization process” indicates, inthe process for producing the first and the second propylene polymerfractions, that the reaction mixture of the first reactor, i.e. thefirst propylene polymer fraction with unreacted monomers, is conveyed,preferably directly conveyed; into a second reactor where a secondpropylene polymer fraction is obtained.

Accordingly, in the sequential polymerization process for producing thefirst and the second propylene polymer fractions, according to theinvention:

-   -   i—the first propylene polymer fraction obtained in the first        reactor generally comprises a first propylene polymer (PP-a)        which is produced in said first reactor,    -   ii—the second propylene polymer fraction obtained in the second        reactor generally comprises a second propylene polymer (PP-b)        which is produced in said second reactor.

Accordingly, the present process comprises at least a first reactor anda second reactor. The process may comprise at least one additionalpolymerization reactor subsequent to the second reactor. In one specificembodiment the process according to the invention consists of twopolymerization reactors i.e. a first reactor and a second reactor. Theterm “polymerization reactor” shall indicate that the mainpolymerization takes place. Thus, in case the process consists of two ormore polymerization reactors, this definition does not exclude theoption that the overall process comprises for instance apre-polymerization step in a pre-polymerization reactor. The term“consists of” is only a closing formulation in view of the mainpolymerization reactors. In case the overall process according to theinvention comprises a pre-polymerization reactor, the term “firstpropylene polymer fraction” means the sum of (co)polymer produced in thepre-polymerization reactor and the (co)polymer produced in the firstreactor.

The reactors are generally selected from slurry and gas phase reactors.

The first reactor is preferably a slurry reactor and can be anycontinuous or simple stirred batch tank reactor or loop reactoroperating in bulk polymerization or slurry polymerization. By “bulkpolymerization” it is meant a process where the polymerization isconducted in a liquid monomer essentially in the absence of an inertdiluent. However, it is known to a person skilled in the art, that themonomers used in commercial production are never pure but always containaliphatic hydrocarbons as impurities. For instance, the propylenemonomer may contain up to 5% of propane as an impurity. Thus, “bulkpolymerization” preferably means a polymerization process in a reactionmedium that comprises at least 60% (wt/wt) of the monomer. According tothe present invention, the first reactor is more preferably a loopreactor.

The second reactor is preferably a gas-phase reactor. Said gas-phasereactor can be any mechanically mixed or fluidized bed reactor orsettled bed reactor. Preferably, the gas-phase reactor comprises amechanically agitated fluidized bed reactor with gas velocities of atleast 0.2 m/sec. The gas-phase reactor of a fluidized bed type reactorcan further include a mechanical agitator to facilitate the mixingwithin the fluidized bed.

The potentially subsequent polymerization reactor or reactors is/arepreferably a gas-phase reactor.

A preferred polymerization process is a “loop-gas phase”-process, suchas developed by Borealis and known as BORSTAR™ technology. Examples ofthis polymerization process are described in EP0887379, WO92/12182,WO2004/000899, WO2004/111095, WO99/24478, WO99/24479 and WO00/68315.

When the overall process according to the invention comprises apre-polymerization reactor, said pre-polymerization step takes placeprior to the polymerization in the first reactor. The pre-polymerizationstep takes place in a pre-polymerization reactor whereinpre-(co)polymerization of propylene is conducted. The pre-polymerizationreactor is of smaller size compared to the first reactor, the secondreactor and the subsequent polymerization reactor or reactors, accordingto the invention, respectively. The reaction volume of thepre-polymerization reactor can be, for example, between 0.001% and 10%of the reaction volume of the first reactor, like the loop reactor. Insaid pre-polymerization reactor, the pre-(co)polymerization of propyleneis performed in bulk or slurry, producing a propylene (co)polymer.

The operating temperature in the pre-polymerization reactor is in therange of 0 to 60° C., preferably in the range of 15 to 50° C., morepreferably in the range of 18 to 35° C.

The pressure in the pre-polymerization reactor is not critical but mustbe sufficiently high to maintain the reaction mixture in liquid phase.Thus, the pressure in the pre-polymerization reactor may be in the rangeof 20 to 100 bar, preferably in the range of 30 to 70 bar.

Hydrogen can be added in the pre-polymerization reactor in order tocontrol the molecular weight, and thus the melt flow rate MFR₂ of thepropylene (co)polymer produced in the pre-polymerization reactor.

In the first reactor of the process according to the invention, amonomer feed comprised of propylene and optionally one or morecomonomers selected from ethylene and C₄-C₁₀ alpha-olefins is fed. Incase the pre-polymerization step is present in the process, thepropylene (co)polymer produced in the pre-polymerization reactor, isalso fed into the first reactor. In the first reactor, a first propylenepolymer fraction is obtained.

The first propylene polymer fraction generally has a comonomer contentselected from ethylene and C₄-C₁₀ alpha-olefins in the range of from 0.0to 1.0 wt %, preferably in the range of from 0.0 to 0.8 wt %, morepreferably in the range of from 0.0 to 0.7 wt %, relative to the totalamount of monomers present in the first propylene polymer fraction.

Generally, the first propylene polymer fraction has a melt flow rate(MFR₂) in the range of from 11 to 60 g/10 min, preferably in the rangeof from 15 to 40 g/10 min, more preferably in the range of from 17 to 35g/10 min. The MFR₂ is determined according to ISO 1133, at a temperatureof 230° C. and under a load of 2.16 kg.

The operating temperature in the first reactor is generally in the rangeof 62 to 85° C., preferably in the range of 65 to 82° C., morepreferably in the range of 67 to 80° C.

Typically, the pressure in the first reactor is in the range of 20 to 80bar, preferably in the range of 30 to 70 bar, more preferably in therange of 35 to 65 bar.

Hydrogen can be added in the first reactor in order to control themolecular weight, and thus the melt flow rate MFR₂ of the firstpropylene polymer fraction obtained in said first reactor.

Generally, the hydrogen/propylene (H₂/C₃) ratio in the first reactor isin the range of 1.5 to 6.0 mol/kmol, preferably in the range of from 1.6to 5.5 mol/kmol, more preferably in the range of from 1.7 to 5.0mol/kmol.

Generally, the ratio of one or more comonomers (selected from ethyleneand C₄-C₁₀ alpha-olefins) to 03 in the first reactor is below 10.0mol/kmol, preferably in the range of from 0.0 to 8.0 mol/kmol, morepreferably in the range of from 0.0 to 7.5 mol/kmol.

Generally, the reaction mixture of the first reactor is conveyed,preferably directly conveyed; into the second reactor. By “directlyconveyed” is meant a process wherein the reaction mixture of the firstreactor is led directly to the next polymerization step, i.e. the secondreactor. Monomers comprising propylene and one or more comonomersselected from ethylene and C₄-C₁₀ alpha-olefins are fed into the secondreactor. In the second reactor, a second propylene polymer fraction isobtained.

The second propylene polymer fraction generally has a comonomer contentselected from ethylene and C₄-C₁₀ alpha-olefins in the range of from 4.5to 20.0 wt %, preferably in the range of from 4.7 to 17.0 wt %, morepreferably in the range of from 4.9 to 15.0 wt %, relative to the totalamount of monomers present in the second propylene polymer fraction.

Generally, the second propylene polymer fraction has a melt flow rate(MFR₂) in the range of from 12 to 60 g/10 min, preferably in the rangeof from 15 to 40 g/10 min, more preferably in the range of from 17 to 35g/10 min. The MFR₂ is determined according to ISO 1133, at a temperatureof 230° C. and under a load of 2.16 kg.

The operating temperature in the second reactor is generally in therange of 70 to 95° C., preferably in the range of 75 to 90° C., morepreferably in the range of 78 to 88° C.

Typically, the pressure in the second reactor is in the range of 5 to 50bar, preferably in the range of 15 to 40 bar.

Hydrogen can be added in the second reactor in order to control themolecular weight, and thus the melt flow rate MFR₂ of the secondpropylene polymer fraction obtained in said second reactor.

Generally, the hydrogen/propylene (H₂/C₃) ratio in the second reactor isin the range of 15.0 to 80.0 mol/kmol, preferably in the range of 17.0to 70.0 mol/kmol, more preferably in the range of 19.0 to 60.0 mol/kmol.

Generally, the ratio of one or more comonomers (selected from ethyleneand C₄-C₁₀ alpha-olefins) to 03 in the second reactor is in the range of45.0 to 200.0 mol/kmol, preferably in the range of 50.0 to 180.0mol/kmol, more preferably in the range of 55.0 to 170.0 mol/kmol.

In the process according to the invention, the propylene polymerproduced in the first reactor i.e. the first propylene polymer (PP-a) isgenerally produced in an amount in the range of from 20 to 90 wt %,preferably in an amount in the range of from 25 to 85 wt %, morepreferably in an amount in the range of from 30 to 80 wt %.

In the process according to the invention, the propylene polymerproduced in the second reactor i.e. the second propylene polymer (PP-b)is generally produced in an amount in the range of from 10 to 80 wt %,preferably in an amount in the range of from 15 to 75 wt %, morepreferably in an amount in the range of from 20 to 70 wt %. The amountof the first propylene polymer (PP-a) and the second propylene polymer(PP-b) is relative to the total sum of first propylene polymer (PP-a)and the second propylene polymer (PP-b) comprised in the secondpropylene polymer fraction.

In the process according to the invention, the one or more comonomersare selected from ethylene and C₄-C₁₀ alpha-olefins, preferably selectedfrom ethylene and C₄-C₈ alpha-olefins, more preferably selected fromethylene and C₄-C₆ alpha-olefins, even more preferably selected from oneor more comonomers comprising ethylene, further even more preferably thecomonomer is selected from solely ethylene, through the presentinvention.

After the polymerization in the second reactor step, the secondpropylene polymer fraction obtained in the second reactor is recoveredby conventional procedures know by the person skilled in the art. Therecovered second propylene polymer fraction according to the inventionis generally in the form of particles.

The propylene polymer (PP-c) is generally produced in a polymerizationprocess, such as slurry polymerization process, gas phase polymerizationprocess or mixture thereof, in the presence of an olefin polymerizationcatalyst.

In a preferred embodiment the propylene polymer (PP-c) is produced in asequential multi-reactor polymerization process. The term “sequentialpolymerization process”, indicates that the propylene polymer (PP-c) isproduced in a process comprising at least two reactors connected inseries. The reactors are generally selected from slurry and gas phasereactors.

A more preferred polymerization process is a “loop-gas phase”-process,such as developed by Borealis and known as BORSTAR™ technology. Examplesof this polymerization process are described in EP0887379, WO92/12182,WO2004/000899, WO2004/111095, WO99/24478, WO99/24479 and WO00/68315.

Generally, the olefin polymerization catalyst is a Ziegler Nattacatalyst. Generally, the polymerization Ziegler Natta catalyst comprisesone or more compounds of a transition metal (TM) of Group 4 to 6 asdefined in IUPAC version 2013, like titanium, further a Group 2 metalcompound, like a magnesium compound and an internal donor (ID).

The components of the catalyst may be supported on a particulatesupport, such as for example an inorganic oxide, like for example silicaor alumina. Alternatively, a magnesium halide may form the solidsupport. It is also possible that the catalyst components are notsupported on an external support, but the catalyst is prepared by anemulsion-solidification method or by a precipitation method, as iswell-known by the man skilled in the art of catalyst preparation.

Preferably, the olefin polymerization catalyst is a specific type ofZiegler-Natta catalyst. In this specific type of Ziegler-Natta catalyst,it is essential that the internal donor is a non-phthalic compound.Preferably, through the whole specific type of Ziegler-Natta catalystpreparation no phthalate compound is used, thus the final specific typeof Ziegler-Natta catalyst does not contain any phthalic compound. Thus,the specific type of Ziegler-Natta catalyst is free of phthaliccompound. Therefore the second propylene polymer fraction and thepropylene polymer (PP-c), produced in the presence of the specific typeof Ziegler-Natta catalyst are free of phthalic compound.

Generally, the specific type of Ziegler-Natta catalyst comprises aninternal donor (ID) which is chosen to be a non-phthalic compound, inthis way the specific type of Ziegler-Natta catalyst is completely freeof phthalic compound. Further, the specific type of Ziegler-Nattacatalyst can be a solid catalyst preferably being free of any externalsupport material, like silica or MgCl₂, and thus the solid catalyst isself-supported.

The solid catalyst is obtainable by the following general procedure:

-   -   a) providing a solution of        -   a₁) at least a Group 2 metal alkoxy compound (Ax) being the            reaction product of a Group 2 metal compound and an            alcohol (A) comprising in addition to the hydroxyl moiety at            least one ether moiety, optionally in an organic liquid            reaction medium; or        -   a₂) at least a Group 2 metal alkoxy compound (Ax′) being the            reaction product of a Group 2 metal compound and an alcohol            mixture of the alcohol (A) and a monohydric alcohol (B) of            formula ROH, optionally in an organic liquid reaction            medium; or        -   a₃) a mixture of the Group 2 metal alkoxy compound (Ax) and            a Group 2 metal alkoxy compound (Bx) being the reaction            product of a Group 2 metal compound and the monohydric            alcohol (B), optionally in an organic liquid reaction            medium; or        -   a₄) Group 2 metal alkoxy compound of formula            M(OR₁)_(n)(OR₂)_(m)X_(2-n-m) or mixture of Group 2 alkoxides            M(OR₁)_(n′)X_(2-n′) and M(OR₂)_(m′)X_(2-m′), where M is a            Group 2 metal, X is halogen, R₁ and R₂ are different alkyl            groups of 2 to 16 carbon atoms, and 0<n′≤2, 0<m′≤2 and            n+m+(2−n−m)=2, provided that n and m are not 0            simultaneously, 0<n′≤2 and 0<m′≤2; and    -   b) adding said solution from step a) to at least one compound of        a transition metal of Group 4 to 6 and    -   c) obtaining the solid catalyst component particles,    -   and adding a non-phthalic internal electron donor (ID) at least        in one step prior to step c).

The internal donor (ID) or precursor thereof is preferably added to thesolution of step a) or to the transition metal compound before addingthe solution of step a).

According to the procedure above, the solid catalyst can be obtained viaa precipitation method or via an emulsion-solidification methoddepending on the physical conditions, especially the temperature used insteps b) and c). An emulsion is also called liquid-liquid two-phasesystem. In both methods (precipitation or emulsion-solidification) thecatalyst chemistry is the same.

In the precipitation method, combination of the solution of step a) withat least one transition metal compound in step b) is carried out and thewhole reaction mixture is kept at least at 50° C., more preferably in atemperature range of 55 to 110° C., more preferably in a range of 70 to100° C., to secure full precipitation of the catalyst component in theform of solid catalyst component particles (step c).

In the emulsion-solidification method, in step b) the solution of stepa) is typically added to the at least one transition metal compound at alower temperature, such as from −10 to below 50° C., preferably from −5to 30° C. During agitation of the emulsion the temperature is typicallykept at −10 to below 40° C., preferably from −5 to 30° C. Droplets ofthe dispersed phase of the emulsion form the active catalystcomposition. Solidification (step c) of the droplets is suitably carriedout by heating the emulsion to a temperature of 70 to 150° C.,preferably to 80 to 110° C. The catalyst prepared by theemulsion-solidification method is preferably used in the presentinvention.

In step a) preferably the solution of a₂) or a₃) is used, i.e. asolution of (Ax′) or a solution of a mixture of (Ax) and (Bx).

Preferably, the Group 2 metal is magnesium. The magnesium alkoxycompounds (Ax), (Ax′), (Bx) can be prepared in situ in the first step ofthe catalyst preparation process, step a), by reacting the magnesiumcompound with the alcohol(s) as described above. Another option is toprepare said magnesium alkoxy compounds separately or they can be evencommercially available as already prepared magnesium alkoxy compoundsand used as such in the catalyst preparation process of the invention.

Illustrative examples of alcohols (A) are glycol monoethers. Preferredalcohols (A) are C₂ to C₄ glycol monoethers, wherein the ether moietiescomprise from 2 to 18 carbon atoms, preferably from 4 to 12 carbonatoms. Preferred examples are 2-(2-ethylhexyloxy) ethanol, 2-butyloxyethanol, 2-hexyloxy ethanol and 1,3-propylene-glycol-monobutyl ether,3-butoxy-2-propanol, with 2-(2-ethylhexyloxy) ethanol and1,3-propylene-glycol-monobutyl ether, 3-butoxy-2-propanol beingparticularly preferred.

The illustrative monohydric alcohol (B) is represented by the structuralformula ROH with R being a straight-chain or branched C₂-C₁₆ alkylresidue, preferably a C₄ to C₁₀ alkyl residue, more preferably a C₆ toC₈ alkyl residue. The most preferred monohydric alcohol is2-ethyl-1-hexanol or octanol.

Preferably, a mixture of Mg alkoxy compounds (Ax) and (Bx) or a mixtureof alcohols (A) and (B), respectively, are used and employed in a moleratio of Bx:Ax or B:A from 10:1 to 1:10, more preferably 6:1 to 1:6,still more preferably 5:1 to 1:3, most preferably 5:1 to 3:1.

The magnesium alkoxy compound may be a reaction product of alcohol(s),as defined above and a magnesium compound selected from dialkylmagnesium, alkyl magnesium alkoxide, magnesium dialkoxide, alkoxymagnesium halide and alkyl magnesium halide. Further, magnesiumdialkoxide, magnesium diaryloxide, magnesium aryloxyhalide, magnesiumaryloxide and magnesium alkyl aryloxide can be used. Alkyl groups in themagnesium compound can be similar or different C₁-C₂₀ alkyl groups,preferably C₂-C₁₀ alkyl groups. Typical alkyl-alkoxy magnesiumcompounds, when used, are ethyl magnesium butoxide, butyl magnesiumpentoxide, octyl magnesium butoxide and octyl magnesium octoxide.Preferably, the dialkyl magnesiums are used. Most preferred, dialkylmagnesiums are butyl octyl magnesium or butyl ethyl magnesium.

It is also possible that the magnesium compound reacts in addition tothe alcohol (A) and alcohol (B) with a polyhydric alcohol (C) of formulaR″(OH)_(m) to obtain said magnesium alkoxide compound. Preferredpolyhydric alcohols, if used, are alcohols, wherein R″ is astraight-chain, cyclic or branched C₂ to C₁₀ hydrocarbon residue and mis an integer of 2 to 6.

The magnesium alkoxy compounds of step a) are thus selected from thegroup consisting of magnesium dialkoxides, diaryloxy magnesiums,alkyloxy magnesium halides, aryloxy magnesium halides, alkyl magnesiumalkoxides, aryl magnesium alkoxides and alkyl magnesium aryloxides or amixture of magnesium dihalide and a magnesium dialkoxide.

The solvent to be employed for the preparation of the present catalystmay be selected from among aromatic and aliphatic straight-chain,branched and cyclic hydrocarbons with 5 to 20 carbon atoms, morepreferably 5 to 12 carbon atoms, or mixtures thereof. Suitable solventsinclude benzene, toluene, cumene, xylol, pentane, hexane, heptane,octane and nonane. Hexanes and pentanes are particularly preferred.

The reaction for the preparation of the magnesium alkoxy compound may becarried out at a temperature of 40 to 70° C. The man skilled in the artknows how to select the most suitable temperature depending on the Mgcompound and alcohol(s) used.

The transition metal (TM) compound of Group 4 to 6 as defined in IUPACversion 2013 is preferably a titanium compound, most preferably atitanium halide, like TiCl₄.

The non-phthalic internal donor (ID) used in the preparation of thespecific type of Ziegler-Natta catalyst used in the present invention ispreferably selected from (di)esters of non-phthalic carboxylic(di)acids, 1,3-diethers, derivatives and mixtures thereof. An especiallypreferred donor is a diester of mono-unsaturated non-phthalicdicarboxylic acids, in particular an ester belonging to a groupcomprising malonates, maleates, succinates, citraconates, glutarates,cyclohexene-1,2-dicarboxylates and benzoates and derivatives thereofand/or mixtures thereof. Preferred examples are e.g. substitutedmaleates and citraconates, most preferably citraconates.

Here and hereinafter the term derivative includes substituted compounds.

In the emulsion-solidification method, the two phase liquid-liquidsystem may be formed by simple stirring and optionally adding (further)solvent(s) and/or additives, such as a turbulence minimizing agent (TMA)and/or an emulsifying agent and/or an emulsion stabilizer, like asurfactant, which are used in a manner known in the art. These solventsand/or additives are used to facilitate the formation of the emulsionand/or stabilize it. Preferably, surfactants are acrylic or methacrylicpolymers. Particularly preferred are unbranched C₁₂ to C₂₀(meth)acrylates such as for example poly(hexadecyl)-methacrylate andpoly(octadecyl)-methacrylate and mixtures thereof. The turbulenceminimizing agent (TMA), if used, is preferably selected from polymers ofα-olefin monomers with 6 to 20 carbon atoms, like polyoctene,polynonene, polydecene, polyundecene or polydodecene or mixturesthereof. Most preferable it is polydecene.

The solid particulate product obtained by the precipitation oremulsion-solidification method may be washed at least once, preferablyat least twice, most preferably at least three times. The washing cantake place with an aromatic and/or aliphatic hydrocarbon, preferablywith toluene, heptane or pentane. Washing is also possible with TiCl₄optionally combined with the aromatic and/or aliphatic hydrocarbon.Washing liquids can also contain donors and/or compounds of Group 13,like trialkyl aluminium, halogenated alky aluminium compounds or alkoxyaluminium compounds. Aluminium compounds can also be added during thecatalyst synthesis. The catalyst can further be dried, for example byevaporation or flushing with nitrogen or it can be slurried to an oilyliquid without any drying step.

The finally obtained specific type of Ziegler-Natta catalyst isdesirably obtained in the form of particles having generally an averageparticle size range of 5 to 200 μm, preferably 10 to 100 μm. Theparticles are generally compact with low porosity and have generally asurface area below 20 g/m², more preferably below 10 g/m². Typically,the amount of Ti present in the catalyst is in the range of 1 to 6 wt %,the amount of Mg is in the range of 10 to 20 wt % and the amount ofinternal donor present in the catalyst is in the range of 10 to 40 wt %of the catalyst composition. A detailed description of the preparationof the catalysts used in the present invention is disclosed inWO2012/007430, EP2610271 and EP2610272.

An external donor (ED) is preferably present as a further component inthe polymerization processes according to the invention. Suitableexternal donors (ED) include certain silanes, ethers, esters, amines,ketones, heterocyclic compounds and blends of these. It is especiallypreferred to use a silane. It is most preferred to use silanes of thegeneral formula (I)R^(a) _(p)R^(b) _(q)Si(OR^(c))_((4-p-q))  (I)wherein R^(a), R^(b) and R^(c) denote a hydrocarbon radical, inparticular an alkyl or cycloalkyl group, and wherein p and q are numbersranging from 0 to 3 with their sum (p+q) being equal to or less than 3.R^(a), R^(b) and R^(c) can be chosen independently from one another andcan be the same or different. Specific examples of silanes according toformula (I) are (tert-butyl)₂Si(OCH₃)₂, (cyclohexyl)(methyl)Si(OCH₃)₂,(phenyl)₂Si(OCH₃)₂ and (cyclopentyl)₂Si(OCH₃)₂. Another most preferredsilane is according to the general formula (II)Si(OCH₂CH₃)₃(NR³R⁴)  (II)wherein R³ and R⁴ can be the same or different and represent a linear,branched or cyclic hydrocarbon group having 1 to 12 carbon atoms. It isin particular preferred that R³ and R⁴ are independently selected fromthe group consisting of methyl, ethyl, n-propyl, n-butyl, octyl,decanyl, iso-propyl, iso-butyl, iso-pentyl, tert.-butyl, tert.-amyl,neopentyl, cyclopentyl, cyclohexyl, methylcyclopentyl and cycloheptyl.Most preferably, ethyl is used.

Generally, in addition to the Ziegler-Natta catalyst or the specifictype of Ziegler-Natta catalyst and the optional external donor (ED) aco-catalyst (Co) can be present in the polymerization processesaccording to the invention. The co-catalyst is preferably a compound ofgroup 13 of the periodic table (IUPAC, version 2013), such as forexample an aluminum compound, e.g., an organo aluminum or aluminumhalide compound. An example of a suitable organo aluminium compound isan aluminum alkyl or aluminum alkyl halide compound. Accordingly, in onespecific embodiment the co-catalyst (Co) is a trialkylaluminium, liketriethylaluminium (TEAL), dialkyl aluminium chloride or alkyl aluminiumdichloride or mixtures thereof. In one specific embodiment theco-catalyst (Co) is triethylaluminium (TEAL).

Generally, the molar ratio between the co-catalyst (Co) and the externaldonor (ED) [Co/ED] and/or the molar ratio between the co-catalyst (Co)and the transition metal (TM) [Co/TM] is carefully chosen for eachprocess. The molar ratio between the co-catalyst (Co) and the externaldonor (ED), [Co/ED] can suitably be in the range of 2.5 to 50.0 mol/mol,preferably in the range of 4.0 to 35.0 mol/mol, more preferably in therange of 5.0 to 30.0 mol/mol. A suitable lower limit can be 2.5 mol/mol,preferably 4.0 mol/mol, more preferably 5.0 mol/mol. A suitable upperlimit can be 50.0 mol/mol, preferably 35.0 mol/mol, more preferably 30.0mol/mol. The lower and upper indicated values of the ranges areincluded.

The molar ratio between the co-catalyst (Co) and the transition metal(TM), [Co/TM] can suitably be in the range of 20.0 to 500.0 mol/mol,preferably in the range of 50.0 to 400.0 mol/mol, more preferably in therange of 100.0 to 300.0 mol/mol. A suitable lower limit can be 20.0mol/mol, preferably 50.0 mol/mol, more preferably 100.0 mol/mol. Asuitable upper limit can be 500.0 mol/mol, preferably 400.0 mol/mol,more preferably 300.0 mol/mol. The lower and upper indicated values ofthe ranges are included.

According to the present invention, the second propylene polymerfraction recovered from the polymerization process is generallymelt-mixed in the presence of additives.

Examples of additives include, but are not limited to, stabilizers suchas antioxidants (for example sterically hindered phenols,phosphites/phosphonites, sulphur containing antioxidants, alkyl radicalscavengers, aromatic amines, hindered amine stabilizers, or blendsthereof), metal deactivators (for example Irganox® MD 1024), or UVstabilizers (for example hindered amine light stabilizers). Othertypical additives are modifiers such as antistatic or antifogging agents(for example ethoxylated amines and amides or glycerol esters), acidscavengers (for example Ca-stearate), blowing agents, cling agents (forexample polyisobutene), lubricants and resins (for example ionomerwaxes, polyethylene- and ethylene copolymer waxes, Fischer Tropschwaxes, montan-based waxes, fluoro-based compounds, or paraffin waxes),as well as slip and antiblocking agents (for example erucamide,oleamide, talc, natural silica and synthetic silica or zeolites) andmixtures thereof.

Following the melt-mixing step, the resulting melt-mixed secondpropylene polymer fraction is pelletized, for example in an underwaterpelletizer or after solidification of one or more strands in a waterbath, in a strand pelletizer.

According to the present invention, the pelletized second propylenepolymer fraction is melt-mixed in the presence of

-   -   i. a propylene polymer (PP-c) containing one or more comonomers        selected from ethylene and C₄-C₁₀ alpha-olefins wherein the        comonomer content is in the range of from 0.5 to 2.5 wt % and    -   ii. at least one alpha-nucleating agent,        in order to produce the inventive polypropylene composition.

The propylene polymer (PP-c) according to the invention generally is apropylene copolymer.

The propylene polymer (PP-c) generally contains one or more comonomersselected from ethylene and C₄-C₁₀ alpha-olefins, preferably selectedfrom ethylene and C₄-C₈ alpha-olefins, more preferably selected fromethylene and C₄-C₆ alpha-olefins, even more preferably selected from oneor more comonomers comprising ethylene, further even more preferably thecomonomer is selected from solely ethylene.

The propylene polymer (PP-c) according to the invention generally has acomonomer content in the range of from 0.5 to 2.5 wt %, preferably inthe range of from 0.6 to 2.0 wt %, more preferably in the range of from0.7 to 1.8 wt %, relative to the total amount of monomers present in thepropylene polymer.

Generally, the propylene polymer (PP-c) has a melt flow rate (MFR₂) inthe range of from 11 to 60 g/10 min, preferably in the range of from 15to 40 g/10 min, more preferably in the range of from 17 to 35 g/10 min.The MFR₂ is determined according to ISO 1133, at a temperature of 230°C. and under a load of 2.16 kg.

The alpha-nucleating agent is generally selected from the groupconsisting of:

-   -   (i) salts of monocarboxylic acids and polycarboxylic acids, e.g.        sodium benzoate or aluminum tert-butylbenzoate,    -   (ii) dibenzylidenesorbitol (e.g. 1,3:2,4 dibenzylidenesorbitol)        and C₁-C₈-alkyl-substituted dibenzylidenesorbitol derivatives,        such as methyldibenzylidenesorbitol, ethyldibenzylidenesorbitol        or dimethyldibenzylidenesorbitol (e.g. 1,3:2,4        di(methylbenzylidene) sorbitol), or substituted        nonitol-derivatives, such as        1,2,3,-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)methylene]-nonitol,    -   (iii) salts of diesters of phosphoric acid, e.g. sodium        2,2′-methylenebis(4,6,-di-tert-butylphenyl) phosphate or        aluminium-hydroxy-bis[2,2′-methylene-bis(4,6-di-t-butylphenyl)phosphate],    -   (iv) vinylcycloalkane polymer and vinylalkane polymer, and    -   (v) mixtures thereof.

Preferably, the alpha-nucleating agent is a dibenzylidenesorbitol (e.g.1,3:2,4 dibenzylidenesorbitol) or a C₁-C₈-alkyl-substituteddibenzylidenesorbitol derivative, such as methyldibenzylidenesorbitol,ethyldibenzylidenesorbitol or dimethyldibenzylidenesorbitol (e.g.1,3:2,4 di(methylbenzylidene) sorbitol) or a substitutednonitol-derivative, such as1,2,3,-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl) methylene]-nonitol.

According to the present invention, generally, 15 to 84 wt % of thepelletized second propylene polymer fraction is melt-mixed in thepresence of 15 to 84 wt % of the propylene polymer (PP-c) and 0.01 to1.0 wt % of the at least one alpha-nucleating agent, based on the totalsum of the pelletized second propylene polymer fraction, the propylenepolymer (PP-c) and the at least one alpha-nucleating agent.

Preferably, 18 to 79.1 wt % of the pelletized second propylene polymerfraction is melt-mixed in the presence of 20 to 81.1 wt % of thepropylene polymer (PP-c) and 0.03 to 0.90 wt % of the at least onealpha-nucleating agent, based on the total sum of the pelletized secondpropylene polymer fraction, the propylene polymer (PP-c) and the atleast one alpha-nucleating agent.

More preferably, 20 to 74.2 wt % of the pelletized second propylenepolymer fraction is melt-mixed in the presence of 25 to 79.2 wt % of thepropylene polymer (PP-c) and 0.05 to 0.80 wt % of the at least onealpha-nucleating agent, based on the total sum of the pelletized secondpropylene polymer fraction, the propylene polymer (PP-c) and the atleast one alpha-nucleating agent.

The melt-mixing of the pelletized second propylene polymer fraction withthe propylene polymer (PP-c) and the at least one alpha-nucleatingagent, is generally carried out in a continuous melt mixing device likefor example an extruder or a co-rotating kneader. The melt mixing devicemay include a feed zone, a kneading zone and a die zone.

Generally, a specific temperature profile is maintained along the screwof the melt-mixing device.

The melt-mixing of the pelletized second propylene polymer with thepropylene polymer (PP-c) and the at least one alpha-nucleating agent, ispreferably carried out in an extruder.

The extruder may be any extruder known in the art. The extruder may thusbe a single screw extruder; a twin screw extruder, such as a co-rotatingtwin screw extruder or a counter-rotating twin screw extruder; or amulti-screw extruder, such as a ring extruder. Preferably, the extruderis a single screw extruder or a twin screw extruder. Especiallypreferred extruder is a co-rotating twin screw extruder.

The extruder typically comprises a feed zone, a melting zone, a mixingzone and a die zone. The extruder, as the person skilled in the artknows, is typically composed by barrels which are comprised in theextruder zones above mentioned.

The extruder typically has a length over diameter ratio, L/D, of up to60:1, preferably of up to 40:1.

Feed Zone

The pelletized second propylene polymer fraction, the propylene polymer(PP-c) and the at least one alpha-nucleating agent are generallyintroduced into the extruder through a feed zone. The feed zone directsthe mixture of the pelletized second propylene polymer fraction, thepropylene polymer (PP-c) and the at least one alpha-nucleating agentinto the melting zone. Typically, the feed zone is formed of a feedhopper and a connection pipe connecting the hopper into the meltingzone. Usually the pelletized second propylene polymer fraction, thepropylene polymer (PP-c) and the at least one alpha-nucleating agentflow through the feed zone under the action of gravity, i.e., generallydownwards.

In a preferred embodiment the at least one alpha-nucleating agent iscompletely introduced or partially introduced via one or more feed portscomprised in the extruder, e.g., via a side feeder.

Melting Zone

Generally, the mixture of the pelletized second propylene polymerfraction, the propylene polymer (PP-c) and the at least onealpha-nucleating agent passes from the feed zone to a melting zone. Inthe melting zone the mixture melts.

Mixing Zone

After the melting zone, the molten mixture passes to a mixing zone. Thescrew in the mixing zone typically comprises one or more mixing sectionswhich comprise screw elements providing a certain degree of backwardflow.

The mixing zone may comprise additional elements, such as a throttlevalve or a gear pump.

Die Zone

The die zone typically comprises a die. No limitation is imposed on thedesign of the die used in the extruder.

In the process according to the invention the pelletized secondpropylene polymer fraction, the propylene polymer (PP-c) and the atleast one alpha-nucleating agent are generally melt-mixed at atemperature in the range of from 190 to 260° C., preferably in the rangeof from 200 to 250° C.

The man skilled in the art is well familiar with the screw speed in theextruder and can easily determine the appropriate screw speed.Generally, the screw speed is adjusted to a range from 100 to 750rotations per minute (rpm), preferably to a range from 150 to 650rotations per minute (rpm).

The extruder may also have one or more feed ports for introducingfurther components, such as for example other polymers or additives,into the extruder. The location of such additional feed ports depends onthe type of material added through the port.

Examples of additives include, but are not limited to, stabilizers suchas antioxidants (for example sterically hindered phenols,phosphites/phosphonites, sulphur containing antioxidants, alkyl radicalscavengers, aromatic amines, hindered amine stabilizers, or blendsthereof), metal deactivators (for example Irganox® MD 1024), or UVstabilizers (for example hindered amine light stabilizers). Othertypical additives are modifiers such as antistatic or antifogging agents(for example ethoxylated amines and amides or glycerol esters), acidscavengers (for example Ca-stearate), blowing agents, cling agents (forexample polyisobutene), lubricants and resins (for example ionomerwaxes, polyethylene- and ethylene copolymer waxes, Fischer Tropschwaxes, montan-based waxes, fluoro-based compounds, or paraffin waxes),as well as slip and antiblocking agents (for example erucamide,oleamide, talc, natural silica and synthetic silica or zeolites) andmixtures thereof.

Generally, the total amount of additives introduced into the extruderduring the melt-mixing of the pelletized second propylene polymerfraction, the propylene polymer (PP-c) and the at least onealpha-nucleating agent, according to the present invention, is not morethan 5.0 wt %, preferably not more than 2.0 wt %, more preferably notmore than 1.5 wt %. The amount of additives is relative to the totalamount of polypropylene composition introduced into the extruder.

At the end of the extruder, a polypropylene composition melt isobtained. The inventive polypropylene composition melt might then bepassed through a die in the optional die zone of the extruder. When theinventive polypropylene composition melt is passed through the die it isgenerally further cooled down and pelletized.

The die zone typically comprises a die plate, which is generally a thickmetal disk having multiple holes. The holes are parallel to the screwaxis.

The pelletizer is generally a strand pelletizer or an underwaterpelletizer.

The invention also provides a polypropylene composition obtainable,preferably obtained, by the process according to the invention.

The polypropylene composition obtainable, preferably obtained, by theprocess according to the invention generally has one or more comonomersselected from ethylene and C₄-C₁₀ alpha-olefins, preferably selectedfrom ethylene and C₄-C₈ alpha-olefins, more preferably selected fromethylene and C₄-C₆ alpha-olefins, even more preferably selected from oneor more comonomers comprising ethylene, further even more preferably thecomonomer is selected from solely ethylene.

The polypropylene composition obtainable, preferably obtained by theprocess according to the invention generally has a comonomer content inthe range of from 1.5 to 5.0 wt %, preferably in the range of from 1.6to 4.0 wt %, more preferably in the range of from 1.7 to 3.5 mol %. Thecomonomer content is relative to the total amount of monomers present inthe polypropylene composition.

Generally, the polypropylene composition obtainable, preferablyobtained, by the process according to the invention has a melt flow rate(MFR₂) in the range of from 12 to 60 g/10 min, preferably in the rangeof from 15 to 40 g/10 min, more preferably in the range of from 17 to 35g/10 min. The MFR₂ is determined according to ISO 1133, at a temperatureof 230° C. and under a load of 2.16 kg.

Generally, the polypropylene composition obtainable, preferablyobtained, by the process according to the invention has a haze value<20%, preferably of from 2% to 18%, more preferably of from 3% to 17%.The haze value is measured according to ASTM D1003 on injection mouldedplaques having 1 mm thickness produced as described in EN ISO 1873-2.

Generally, the polypropylene composition obtainable, preferablyobtained, by the process according to the invention has a meltingtemperature >152° C., preferably in the range of from 153 to 163° C.,more preferably in the range of 154 to 162° C., even more preferably inthe range of from 154 to 160° C. The melting temperature (Tm) ismeasured by DSC according to ISO 11357/3.

Generally, the polypropylene composition obtainable, preferablyobtained, by the process according to the invention has acrystallization temperature >120° C., preferably in the range of from122 to 132° C., more preferably in the range of 123 to 130° C. Themelting temperature (Tc) is measured by DSC according to ISO 11357/3.

Generally, the polypropylene composition obtainable, preferablyobtained, by the process according to the invention has a xylene solublecontent (XCS) in the range of 5.5 to 18.0 wt %, preferably in the rangeof from 6.0 to 16.0 wt %, more preferably in the range of from 6.2 to15.0 wt %. The xylene soluble fraction is determined according to ISO16152 at 25° C.

Generally, the polypropylene composition obtainable, preferablyobtained, by the process according to the invention has a tensilemodulus >950 MPa, preferably in the range of from 951 to 1600 MPa, morepreferably in the range of from 1000 to 1600 MPa, even more preferablyin the range of from 1050 to 1550 MPa. The tensile modulus is measuredaccording to ISO 527-1:2012/ISO 527-2:2012 at 23° C. on injectionmoulded test specimens.

Generally, the polypropylene composition obtainable, preferablyobtained, by the process according to the invention has a Charpy notchedimpact strength >4.8 kJ/m², preferably in the range of from 4.9 to 20.0kJ/m², more preferably in the range of from 5.0 to 15.0 kJ/m², even morepreferably in the range of from 5.0 to 13.0 kJ/m², even further morepreferably in the range of from 7.0 to 13 kJ/m². The Charpy notchedimpact strength is measured according to ISO 179/1eA at 23° C. oninjection moulded test specimens as described in EN ISO 1873-2.

The present invention also provides an article comprising thepolypropylene composition obtainable, preferably obtained, by theprocess according to the invention. Suitable articles are films, likefor example cast films, and injection moulded articles. A preferredarticle is a closure cap, a screw cap or a closure system for food orfluid packaging.

Finally, the present invention relates to the use of the polypropylenecomposition obtainable, preferably obtained by the process according tothe invention in the preparation of a cast film or an injection mouldedarticle, preferably a closure cap, a screw cap or a closure system forfood or fluid packaging. A fluid is commonly defined as a substance thatcontinually deforms (flows) under an applied shear stress.

EXAMPLES I. Measuring Methods

a) Melt Flow Rate

The melt flow rate (MFR) is determined according to ISO 1133 and isindicated in g/10 min. The MFR is an indication of the flowability andhence the processability of the polymer. The higher the melt flow rate,the lower the viscosity of the polymer. The MFR₂ of polypropylene isdetermined at a temperature of 230° C. and under a load of 2.16 kg.

b) DSC Analysis

The melting temperature and the crystallisation temperature are measuredwith a TA Instrument Q2000 differential scanning calorimetry device(DSC) according to ISO 11357/3 on 5 to 10 mg samples, under 50 mL/min ofnitrogen atmosphere. Crystallisation and melting temperatures wereobtained in a heat/cool/heat cycle with a scan rate of 10° C./minbetween 30° C. and 225° C. Crystallisation and melting temperatures weretaken as the peaks of the endotherms and exotherms in the cooling stepand the second heating step respectively.

c) Xylene Soluble Content (XCS, wt %)

The content of the polymer soluble in xylene is determined according toISO 16152; 5^(th) edition; 2005 Jul. 1 at 25° C.

d) Tensile Modulus

Tensile Modulus is measured according to ISO 527-1:2012/ISO527-2:2012 at23° C. and at a cross head speed=50 mm/min; using injection moulded testspecimens as described in EN ISO 1873-2 (dog bone shape, 4 mmthickness).

e) Charpy Notched Impact

Charpy notched impact strength is determined according to ISO 179/1eA at23° C. on injection moulded test specimens as described in EN ISO 1873-2(80×10×4 mm).

f) Haze

Haze is determined according to ASTM D1003 on injection moulded plaqueshaving 1 mm 35 thickness and 60×60 mm² area produced as described in ENISO 1873-2.

g) Comonomer Content

Poly(Propylene-Co-Ethylene)-Ethylene Content by IR Spectroscopy

Quantitative infrared (IR) spectroscopy was used to quantify theethylene content of the poly(ethylene-co-propene) copolymers throughcalibration to a primary method.

Calibration was facilitated through the use of a set of in-housenon-commercial calibration standards of known ethylene contentsdetermined by quantitative ¹³C solution-state nuclear magnetic resonance(NMR) spectroscopy. The calibration procedure was undertaken in theconventional manner well documented in the literature. The calibrationset consisted of 38 calibration standards with ethylene contents ranging0.2-75.0 wt % produced at either pilot or full scale under a variety ofconditions. The calibration set was selected to reflect the typicalvariety of copolymers encountered by the final quantitative IRspectroscopy method.

Quantitative IR spectra were recorded in the solid-state using a BrukerVertex 70 FTIR spectrometer. Spectra were recorded on 25×25 mm squarefilms of 300 um thickness prepared by compression moulding at 180-210°C. and 4-6 mPa. For samples with very high ethylene contents (>50 mol %)100 um thick films were used. Standard transmission FTIR spectroscopywas employed using a spectral range of 5000-500 cm⁻¹, an aperture of 6mm, a spectral resolution of 2 cm⁻¹, 16 background scans, 16 spectrumscans, an interferogram zero filling factor of 64 and Blackmann-Harris3-term apodisation.

Quantitative analysis was undertaken using the total area of the CH₂rocking deformations at 730 and 720 cm⁻¹ (A_(Q)) corresponding to(CH₂)_(>2) structural units (integration method G, limits 762 and 694cm⁻¹). The quantitative band was normalised to the area of the CH bandat 4323 cm⁻¹ (A_(R)) corresponding to CH structural units (integrationmethod G, limits 4650, 4007 cm⁻¹). The ethylene content in units ofweight percent was then predicted from the normalised absorption(A_(Q)/A_(R)) using a quadratic calibration curve. The calibration curvehaving previously been constructed by ordinary least squares (OLS)regression of the normalised absorptions and primary comonomer contentsmeasured on the calibration set.

Poly(Propylene-Co-Ethylene)-Ethylene Content for Calibration Using ¹³CNMR Spectroscopy

Quantitative ¹³C{¹H} NMR spectra were recorded in the solution-stateusing a Bruker Avance III 400 NMR spectrometer operating at 400.15 and100.62 MHz for ¹H and ¹³C respectively. All spectra were recorded usinga ¹³C optimised 10 mm extended temperature probehead at 125° C. usingnitrogen gas for all pneumatics. Approximately 200 mg of material wasdissolved in 3 ml of 1,2-tetrachloroethane-d₂ (TCE-d₂) along withchromium (III) acetylacetonate (Cr(acac)₃) resulting in a 65 mM solutionof relaxation agent in solvent (Singh, G., Kothari, A., Gupta, V.,Polymer Testing 28 5 (2009), 475). To ensure a homogenous solution,after initial sample preparation in a heat block, the NMR tube wasfurther heated in a rotatory oven for at least 1 hour. Upon insertioninto the magnet the tube was spun at 10 Hz. This setup was chosenprimarily for the high resolution and quantitatively needed for accurateethylene content quantification. Standard single-pulse excitation wasemployed without NOE, using an optimised tip angle, 1 s recycle delayand a bi-level WALTZ16 decoupling scheme (Zhou, Z., Kuemmerle, R., Qiu,X., Redwine, D., Cong, R., Taha, A., Baugh, D. Winniford, B., J. Mag.Reson. 187 (2007) 225, Busico, V., Carbonniere, P., Cipullo, R.,Pellecchia, R., Severn, J., Talarico, G., Macromol. Rapid Commun. 2007,28, 1128). A total of 6144 (6k) transients were acquired per spectra.Quantitative ¹³C{¹H} NMR spectra were processed, integrated and relevantquantitative properties determined from the integrals. All chemicalshifts were indirectly referenced to the central methylene group of theethylene block (EEE) at 30.00 ppm using the chemical shift of thesolvent. This approach allowed comparable referencing even when thisstructural unit was not present. Characteristic signals corresponding tothe incorporation of ethylene were observed (Cheng, H. N.,Macromolecules 17 (1984), 1950) and the comonomer fraction calculated asthe fraction of ethylene in the polymer with respect to all monomer inthe polymer: fE=(E/(P+E) The comonomer fraction was quantified using themethod of Wang et. al. (Wang, W-J., Zhu, S., Macromolecules 33 (2000),1157) through integration of multiple signals across the whole spectralregion in the ¹³C{¹H} spectra. This method was chosen for its robustnature and ability to account for the presence of regio-defects whenneeded. Integral regions were slightly adjusted to increaseapplicability across the whole range of encountered comonomer contents.For systems with very low ethylene content where only isolated ethylenein PPEPP sequences were observed the method of Wang et. al. was modifiedreducing the influence of integration of sites that are no longerpresent. This approach reduced the overestimation of ethylene contentfor such systems and was achieved by reduction of the number of sitesused to determine the absolute ethylene content toE=0.5(Sββ+Sβγ+Sβδ+0.5(Sαβ+Sαγ)) Through the use of this set of sites thecorresponding integral equation becomesE=0.5(I_(H)+I_(G)+0.5(I_(C)+I_(D))) using the same notation used in thearticle of Wang et. al. (Wang, W-J., Zhu, S., Macromolecules 33 (2000),1157). Equations used for absolute propylene content were not modified.The mole percent comonomer incorporation was calculated from the molefraction: E [mol %]=100*fE. The weight percent comonomer incorporationwas calculated from the mole fraction: E [wt%]=100*(fE*28.06)/((fE*28.06)+((1−fE)*42.08)).

II. Inventive and Comparative Examples a) Catalyst Preparation

For the preparation of the catalyst 3.4 litre of 2-ethylhexanol and 810ml of propylene glycol butyl monoether (in a molar ratio 4/1) were addedto a 20.0 l reactor. Then 7.8 litre of a 20.0% solution in toluene ofBEM (butyl ethyl magnesium) provided by Crompton GmbH, were slowly addedto the well stirred alcohol mixture. During the addition, thetemperature was kept at 10.0° C. After addition, the temperature of thereaction mixture was raised to 60.0° C. and mixing was continued at thistemperature for 30 minutes. Finally after cooling to room temperaturethe obtained Mg-alkoxide was transferred to a storage vessel.

21.2 g of Mg alkoxide prepared above was mixed with 4.0 mlbis(2-ethylhexyl) citraconate for 5 min. After mixing the obtained Mgcomplex was used immediately in the preparation of the catalystcomponent.

19.5 ml of titanium tetrachloride was placed in a 300 ml reactorequipped with a mechanical stirrer at 25.0° C. Mixing speed was adjustedto 170 rpm. 26.0 g of Mg-complex prepared above was added within 30minutes keeping the temperature at 25.0° C. 3.0 ml of Viscoplex® 1-254and 1.0 ml of a toluene solution with 2 mg Necadd 447™ was added. Then24.0 ml of heptane was added to form an emulsion. Mixing was continuedfor 30 minutes at 25.0° C., after which the reactor temperature wasraised to 90.0° C. within 30 minutes. The reaction mixture was stirredfor a further 30 minutes at 90.0° C. Afterwards stirring was stopped andthe reaction mixture was allowed to settle for 15 minutes at 90.0° C.The solid material was washed 5 times: washings were made at 80.0° C.under stirring for 30 min with 170 rpm. After stirring was stopped thereaction mixture was allowed to settle for 20-30 minutes and followed bysiphoning.

-   -   Wash 1: washing was made with a mixture of 100 ml of toluene and        1 ml donor    -   Wash 2: washing was made with a mixture of 30 ml of TiCl₄ and 1        ml of donor.    -   Wash 3: washing was made with 100 ml of toluene.    -   Wash 4: washing was made with 60 ml of heptane.    -   Wash 5: washing was made with 60 ml of heptane under 10 minutes        stirring.

Afterwards stirring was stopped and the reaction mixture was allowed tosettle for 10 minutes while decreasing the temperature to 70° C. withsubsequent siphoning, followed by N₂ sparging for 20 minutes to yield anair sensitive powder.

b) Inventive Examples (IE1, IE2 and IE3)

The second propylene polymer fraction as well as the propylene polymer(PP-c) related to the inventive examples (IE1, IE2 and IE3) wereproduced in a pilot plant with a prepolymerization reactor, one slurryloop reactor and one gas phase reactor. The solid catalyst componentdescribed above along with triethyl-aluminium (TEAL) as co-catalyst anddicyclo pentyl dimethoxy silane (D-donor) as external donor, were usedin the preparation of the second propylene polymer fraction and thepropylene polymer (PP-c), respectively.

The polymerization process conditions and properties of the secondpropylene polymer fraction and the propylene polymer (PP-c) aredescribed in Table 1.

The inventive polypropylene compositions were prepared by extruding therespective amount of second propylene polymer fraction, of propylenepolymer (PP-c) and of nucleating agent in a co-rotating twin screwextruder type Coperion ZSK 40 (screw diameter 40 mm, L/D ratio 38). Thetemperatures in the extruder were in the range of 190-230° C. In each ofthe inventive examples 0.05 wt % of Irganox 1010(Pentaerythrityl-tetrakis(3-(3′,5′-di-tert.butyl-4-hydroxyphenyl)-propionate, CAS No. 6683-19-8, commerciallyavailable from BASF AG, Germany), 0.05 wt % of Irgafos 168 (Tris(2,4-di-t-butylphenyl) phosphite, CAS No. 31570-04-4, commerciallyavailable from BASF AG, Germany), 0.10 wt % of Calcium stearate (CAS.No. 1592-23-0, commercially available under the trade name Ceasit Flfrom Baerlocher GmbH, Germany) and 0.06 wt % of Glycerol monostearate(CAS No. 97593-29-8, commercially available with 90% purity under thetrade name Grindsted PS 426 from Danisco NS, Denmark) were added to theextruder as additives.

Following the extrusion step and after solidification of the strands ina water bath, the resulting polypropylene composition was pelletized ina strand pelletizer.

The polypropylene composition properties are described in Table 2.

c) Comparative Examples (CE1 and CE2)

CE-1 is a C₂ propylene random copolymer having an MFR₂ of 13.0 g/10 min,produced in one reactor process and distributed by Borealis under theTrade name RE420MO.

CE-2 is a C₂ propylene random copolymer having an MFR₂ of 20.0 g/10 min,produced in one reactor process and distributed by Borealis under theTrade name RF365MO.

TABLE 1 Preparation and properties of the second propylene polymerfraction and the propylene polymer (PP-c). Second propylene Propylenepolymer fraction polymer (PP-c) Pre-polymerization reactor Temperature[° C.] 30 30 Catalyst feed [g/h] 3.0 2.9 D-Donor [g/t propylene] 41.141.1 TEAL/propylene [g/t propylene] 170 170 Al/D-Donor [Co/ED] [mol/mol]8.3 8.2 Al/Ti [Co/TM] [mol/mol] 217 216 Residence Time [h] 0.3 0.3 Loopreactor Temperature [° C.] 70 70 Pressure [kPa] 5270 5380 Residence time[h] 0.4 0.4 Split [%] 56 64 H₂/C₃ ratio [mol/kmol] 1.7 1.8 C₂/C₃ ratio[mol/kmol] 3.1 3.1 MFR₂ [g/10 min] 19 21 C₂ content [wt %] 0.5 0.5Gas-phase reactor Temperature [° C.] 80 80 Pressure [kPa] 1900 1820Residence time [h] 1.4 1.4 Split [%] 44 36 H₂/C₃ ratio [mol/kmol] 45.927.4 C₂/C₃ ratio [mol/kmol] 62.9 8.5 MFR₂ [g/10 min] 20 22 C₂ content[wt %] 4.5 1.0 *Split relates to the amount of propylene polymerproduced in each specific reactor.

TABLE 2 Extrusion process conditions and polypropylene compositionproperties. 1E1 1E2 1E3 CE1 CE2 Second propylene [wt %] 59.6 42.0 22.00.0 0.0 polymer fraction Propylene polymer [wt %] 40.0 57.6 77.6 0.0 0.0(PP-c) Nucleating agent [wt %] 0.2 0.2 0.2 0.0 0.0 (Millad 3988 ®)Composition properties* MFR₂ [g/10 min] 21.0 20.0 22.0 13.0 20.0 C₂content [wt %] 3.0 2.4 1.7 3.4 3.4 XCS [wt %] 11.4 9.1 6.5 5.8 6.8Tensile Modulus [MPa] 1256 1397 1573 921 1138 Charpy notched [kJ/m²] 7.05.6 4.9 5.3 4.5 impact strength 23° C. Haze (1 mm) [%] 17 18 18 23 20 Tm[° C.] 160 160 160 150 151 Tc [° C.] 127 128 127 120 120 *measured onpellets obtained after the extrusion process.

From Table 2 it can be derived that the polypropylene compositions(inventive examples) show an improved balanced combination of highflowability, high stiffness and impact, and high level of opticalproperties (low haze value), compared to the comparative examples.

The invention claimed is:
 1. A process for producing a polypropylenecomposition by sequential polymerization, the process comprising thesteps of: a) polymerizing in a first reactor monomers comprisingpropylene and optionally one or more comonomers selected from ethyleneand C₄-C₁₀ alpha-olefins, to obtain a first propylene polymer fractionhaving a comonomer content in the range of 0.0 to 1.0 wt %, b)polymerizing in a second reactor monomers comprising propylene and oneor more comonomers selected from ethylene and C₄-C₁₀ alpha-olefins, inthe presence of the first propylene polymer fraction to obtain a secondpropylene polymer fraction having a comonomer content in the range offrom 4.5 to 20.0 wt %, c) pelletizing the second propylene polymerfraction, and d) melt-mixing the pelletized second propylene polymerfraction in the presence of: i. a propylene polymer (PP-c) produced in aprocess comprising at least two reactors connected in series, thepropylene polymer (PP-c) containing one or more comonomers selected fromethylene and C₄-C₁₀ alpha-olefins wherein the comonomer content is inthe range of from 0.5 to 2.5 wt %, and ii. at least one alpha-nucleatingagent, wherein the polypropylene composition has: i—an MFR₂ in the rangeof from 11.0 to 60.0 g/10 min, as measured according to ISO 1133 at 230°C. under a load of 2.16 kg; ii—a haze value of <20%, as measuredaccording to ASTM D1003 on injection moulded plaques having 1 mmthickness produced as described in EN ISO 1873-2; iii—a tensilemodulus >950 MPa, as measured according to ISO 527-1:2012/ISO 527-2:2012on an injection moulded test specimen at 23° C.; iv—15 to 84 wt % of thepelletized second polymer fraction relative to the total amount ofpolypropylene composition; v—15 to 84% of the propylene polymer (PP-c)relative to the total amount of polypropylene composition; and vi—0.01to 1.0 wt % of at least one alpha-nucleating agent relative to the totalamount of polypropylene composition.
 2. The process according to claim1, wherein the polypropylene composition has a comonomer content in therange of from 1.5 to 5.0 wt %, relative to the total amount of monomerspresent in the polypropylene composition.
 3. The process according toclaim 1, wherein the melt-mixing of the pelletized second propylenepolymer fraction is carried out at a temperature in the range of 190° C.to 260° C.
 4. The process according to claim 1, wherein the melt-mixingof the pelletized second propylene polymer fraction is carried out in asingle-screw extruder or a twin-screw extruder.
 5. The process accordingto claim 1, wherein the second propylene polymer fraction and thepropylene polymer (PP-c) are free of a phthalic compound.
 6. The processaccording to claim 1, wherein the first reactor is a slurry reactor. 7.The process according to claim 1, wherein the second reactor is agas-phase reactor.
 8. A polypropylene composition obtainable by theprocess according to claim
 1. 9. The polypropylene composition accordingto claim 8, wherein the polypropylene composition has a xylene solublecontent (XCS) in the range of from 5.5 to 18.0 wt %, as determinedaccording to ISO 16152 at 25° C.
 10. The polypropylene compositionaccording to claim 8, wherein the polypropylene composition has amelting temperature >152° C. and a crystallization temperature >120° C.as measured according to ISO 11357/3.
 11. The polypropylene compositionaccording to claim 8, wherein the polypropylene composition has a Charpynotched impact strength >4.8 kJ/m², as measured according to ISO 179/1eAat 23° C. on injection moulded test specimens as described in EN ISO1873-2.
 12. The polypropylene composition according to claim 8, whereinthe polypropylene composition is free of a phthalic compound.
 13. Anarticle comprising the polypropylene composition according to claim 8.14. The article according to claim 13, wherein the article is a castfilm, an injection moulded article, a closure cap, a screw cap, or aclosure system for food or fluid packaging.